1. Field of the Invention
The present invention relates to a shakeproof camera having a shake correction function.
2. Description of the Related Art
Video input devices such as television cameras, electronic still cameras, video cameras, and industrial image measuring devices have been very popular. Camera shake cause problems in the use of these video input devices. Camera shake causes not only degradation of a photographed image, but also an erroneous operation of a control system for automatic focus control or the like.
A camera shake correction apparatus comprises a shake detecting means for detecting a camera shake and a correcting means for correcting an image in accordance with the amount and direction of a shake detected by the shake detecting means. The shake detecting means employs a method of causing an acceleration sensor or angular velocity sensor to physically detect a shake or an electronic method of processing a photographed image signal to detect a movement vector. The shake correcting means employs a method of optically correcting the shake by means of a variable vertical angle prism or an electronic method for selecting a read range using an image memory.
An image pickup apparatus for electronically detecting a camera shake and optically correcting the camera shake is described in Japanese Laid-Open Patent Application No. 61-269572, and the block diagram of its schematic arrangement is shown in FIG. 1.
A variable vertical angle prism 10 can change a photographic optical axis in the pitch and yaw directions. A beam from an object to be photographed is incident on a photoelectric conversion surface of a CCD image pickup element 14 through the variable vertical angle prism 10 and a zoom lens 12. The image pickup element 14 converts an optical image of the object into an electrical signal. A process circuit 16 converts an output from the image pickup element 14 into a video signal. The variable vertical angle prism is arranged such that a liquid having a high refractive index is sealed between two parallel transparent glass plates, and the relative angle between the two glass plates is set to be variable to obtain a variable vertical angle.
A movement vector detection circuit 18 compares output signals representing a current frame and a previous frame from the process circuit 16 and detects an amount of movement of the object (or the camera) and its direction, i.e., the movement vector. A control circuit 20 uses predetermined coefficients to perform convolution processing of the movement vector detected by the movement vector detection circuit 18. The control circuit 20 generates a drive signal for the variable vertical angle prism 10 and supplies it to an actuator 22. The actuator 22 drives the variable vertical angle prism 10 in accordance with the drive signal from the control circuit 20. The variable vertical angle prism 10 shifts the photographic optical axis in the pitch and yaw directions so as to compensate for or cancel the image shake caused by the camera shake.
More specifically, when the entire apparatus vibrates during photography of an object, the angle of incident light from the object changes, and a position X.sub.0 of the object image on the image pickup surface changes accordingly. The movement vector detection circuit 18 outputs a difference between the object image position of the previous frame and that of the current frame. This difference signal is calculated by the control circuit 20, thereby driving the variable vertical angle prism 10.
The exposure time (accumulation time) and the period of the CCD image pickup element 14 are variable. For example, if an object which is moving at a high speed is to be photographed, a high-speed (e.g., 1/1,000 sec.) shutter mode is set to prevent blurring of the edge of the object image. If an object has a low luminance, a long exposure mode is set in which the exposure period is set longer than the field period so as to obtain a sufficiently high S/N ratio. The exposure time and the exposure period of the image pickup element 14 are controlled by control signals from a timing control circuit 24. The exposure time and the exposure period can be selected by a means such as an operation switch 26. That is, when an operation mode of the image pickup element 14 is designated with the operation switch 26, the timing control circuit 24 drives the image pickup element 14 in accordance with the above designation.
Even in a long exposure mode wherein the exposure time exceeds the field period, the process circuit 16 must output a video signal of a standard field period. For this reason, the output from the timing control circuit 24 is also supplied to the process circuit 16. In the long exposure mode, the process circuit 16 repeatedly outputs the video signal of the same frame during exposure of the image pickup element 14.
The above-described camera shake correction system is a feedback system. The response characteristics of all the elements of the feedback loop, i.e., the variable vertical angle prism 10, the zoom lens 12, the image pickup element 14, the process circuit 16, the movement vector detection circuit 18, the control circuit 20, and the actuator 22 at an object position X.sub.0 within the frame determine the shakeproof characteristics of the camera. This system is a discrete time system having a frame output cycle (normally a vertical frequency of 60 Hz) of the image pickup element 14 as a sampling frequency.
The response characteristics, and particularly idle time components of the image pickup element 14, the process circuit 16, and the movement vector detection circuit 18 are taken into consideration. In the normal mode, the image pickup element 14 accumulates the charge signal of the object image focused on the light-receiving surface for a one-field period and outputs the accumulated charge signal in the next field period. The movement vector detection circuit 18 starts a detection operation in synchronism with the read operation of the charge from the image pickup element 14 (or the start of the image portion of the video signal output from the process circuit 16). The movement vector detection circuit 18 outputs a detection result in the vertical blanking period.
If the image movement caused by the camera shake has an average value at the center during the charge accumulation period of the image pickup element 14, an idle time of 1.5 field periods is caused in the section consisting of the image pickup element 14, the process circuit 16, and the movement vector detection circuit 18. Note that the delay time in the process circuit 16 is neglected. The idle time is long not to be neglected with respect to the sampling frequency. The variation in idle time greatly influences the shakeproof performance.
As described above, in the CCD image pickup element, the exposure time and the exposure period can be externally controlled. However, the characteristics of the shakeproof control system are set so that a camera shake such as a hand shake can be effectively suppressed in the normal photographic mode. FIG. 2 shows the frequency characteristics of the shakeproof characteristics. The gain is plotted along the ordinate, and the frequency is plotted along the abscissa. Since the hand shake in the camera has a strong spectral component near 1 Hz, the control circuit 20 is set to intensively suppress the component near 1 Hz, as indicated by a characteristic curve 30.
When the control circuit 20 is set under the above conditions, and the image pickup element 14 is operated in a high-speed shutter mode, the overall characteristics are shifted to the high frequency side, as indicated by a characteristic curve 32. In the long exposure mode, the overall characteristics are shifted to the low frequency side, as indicated by a characteristic curve 34. In either case, the suppression performance near 1 Hz is degraded. That is, in the shakeproof camera using the image pickup element, the shakeproof characteristics change in accordance with changes in operation modes of the image pickup element. As a result, appropriate camera shake suppression cannot be performed.